A torque demand control method for determining an operation amount of each actuator by using torque as a control amount is known as a method of controlling an internal combustion engine. An example of a control device for providing torque demand control is described in JP-A-2009-068430. This control device (hereinafter referred to as the conventional control device) provides torque control by exercising air amount control with a throttle and exercising ignition timing control with an ignition device. The conventional control device determines a target air amount in accordance with a target torque and calculates a throttle opening from the target air amount by using an inverse model of an air model. Further, the conventional control device uses the air model to calculate an estimated air amount that is attained by a current throttle opening, and then calculates an estimated torque from the estimated air amount. Subsequently, the conventional control device determines an ignition timing retard amount in accordance with the difference between the target torque and the estimated torque.
Meanwhile, not only the intake air amount and ignition timing but also the air-fuel ratio is closely related to torque generated by a spark-ignition internal combustion engine. Therefore, the intake air amount, fuel injection amount, and ignition timing are controlled in accordance with the target torque and target air-fuel ratio as described, for instance, in JP-A-11-82090. There is also a known technology for controlling the air-fuel ratio in accordance with the magnitude of torque generated by an internal combustion engine as described in JP-A-9-240322.
Further, there is a well-known technology for positively controlling the air-fuel ratio. This technology provides enhanced catalytic conversion efficiency by causing the air-fuel ratio to periodically vary around a stoichiometric value. If, in this instance, the torque periodically varies with the air-fuel ratio, driveability is impaired by a noticeable torque change. It is therefore necessary to devise a scheme for periodically varying only the air-fuel ratio while maintaining the torque constant.
The conventional control device has an air amount map for determining the target air amount from the target torque, and uses the air-fuel ratio as a map search key. Therefore, when the air-fuel ratio periodically varies, the target air amount also periodically varies with the air-fuel ratio. This causes the throttle opening to be controlled accordingly. In this instance, the throttle moves in such a manner that an increase/decrease in the air amount offsets torque variations caused by air-fuel ratio oscillation. More specifically, when the air-fuel ratio becomes richer, the throttle moves in a closing direction so that a resulting increase in the torque is canceled by a decrease in the air amount. When, in contrast, the air-fuel ratio becomes leaner, the throttle moves in an opening direction so that a resulting decrease in the torque is canceled by an increase in the air amount.
However, the air amount responds to a throttle movement with a delay. Therefore, the actual air amount changes with a delay in response to a change in the target air amount. Consequently, when the air-fuel ratio periodically oscillates, a phase difference arises between actual air amount changes and air-fuel ratio changes. To let air amount changes offset torque variations caused by air-fuel ratio oscillation, it is necessary that the air amount changes and air-fuel ratio changes be in opposite phase with each other. However, it is difficult for the conventional control device described above to completely eliminate torque variations caused by air-fuel ratio oscillation because there arises a phase difference between the air amount changes and air-fuel ratio changes.
As is obvious from the above, the conventional control device needs further improvement to achieve a target torque in a situation where the air-fuel ratio periodically varies.